Treatment of the neonate with the neurocytotoxin 6-OHDA leads to a decrease in forebrain catecholamine content with a concomitant increase in pontine and medullary levels. The proposed experiments will test the hypothesis that the increased catecholamine level in the lower brainstem reflects collateral sprouting plasticity in branches of the axonal plexus of noradrenergic neurons. Since one of the main descending projections from the locus coeruleus consists of a fiber bundle passing along the dorsomedial border of the trigenimal nuclear complex, and locus coeruleus stimulation leads to an inhibition in trigeminal-thalamic relay neurons, the hypothesis will be tested that hypercollateralization of noradrenergic fibers produces increased postsynaptic inhibition following locus coeruleus activation without affecting the presynaptic inhibition evoked from the sensory cortex. Anatomical studies will determine the distributon of the terminal axonal fields of locus coeruleus neurons in animals treated with neonatal 6-OHDA. Of particular interest is the question of whether hypercollateralization of locus coeruleus axons results in formation of new terminal branches in regions which do not ordinarily receive a monaminergic input. The possibility of regeneration in the ventral tier thalamic nuclei ventralis anterior and ventralis lateralis will be studied also. The proposed experiments will involve measurement of catecholamine levels, glyoxylic acid induced histofluorescence, and single unit electrophysiological recordings.